The present invention is directed to a gate valve. More particularly, the invention is directed to a manually actuated, rising stem gate valve which includes a roller screw assembly to efficiently convert rotation of the handwheel into translation of the valve stem.
Gate valves are used in a variety of industries to control the flow of fluids. In particular, gate valves are used extensively in the oil and gas industry to control the flow of produced fluids at various stages of production. Most gate valves used in this industry comprise a valve body having a longitudinal flow bore and a transverse gate cavity that intersects the flow bore. A gate having a gate opening extending transversely therethrough is disposed in the gate cavity. A stem is provided for moving the gate between an open position, in which the gate opening is aligned with the flow bore, and a closed position, in which the gate opening is offset from the flow bore. The gate is usually positioned between a pair of seats, each of which seals against the gate under pressure to prevent fluid from passing through the flow bore when the gate is in the closed position.
The gate cavity is normally covered by a bonnet having an axial through bore. The stem passes through the through bore and is sealed to the bonnet by a stem packing to contain the fluid pressure within the gate cavity. Many gate valves are also provided with a backseat mechanism, that is, cooperating sealing surfaces on the stem and the bonnet which are located below the stem packing. Often a desire exists to perform maintenance or repair on the gate valve, such as replacing the stem packing, without removing the gate valve from the conduit system to which it is connected. In such instances, the stem is moved upwardly until the backseat sealing surfaces on the stem and the bonnet engage and form a metal-to-metal seal. This backseating procedure thus isolates the stem from the gate cavity and allows the desired maintenance to be performed without having to remove the gate valve from the conduit system. For safety reasons, the pressure in the gate cavity is bled down to ambient pressure before any maintenance is performed. In addition, any residual pressure between the stem packing and the backseat is usually bled off through a bleeder plug provided in the bonnet.
Gate valves are provided with means for manipulating the stem to raise and lower of the gate. In this respect, gate valves may be divided into two groups: (a) rising stem gate valves and (b) non-rising stem gate valves. In a non-rising (or rotating) stem gate valve, the stem is threadedly connected to the gate such that rotation of the stem causes the gate to move up and down. An actuation mechanism is provided for selectively rotating the stem clockwise or counterclockwise in order to open or close the gate valve. On this type of gate valve, the backseat is set by driving the gate down until it bottoms out on the valve body, and then allowing the stem to move upward until it backseats against the bonnet. Such valves may be automatically or remotely actuated, such as by an electric motor. Alternatively, these gate valves may be manually actuated, such as by a handwheel adapted to rotate the stem directly. An example of such a manual gate valve is shown in U.S. Pat. No. 5,762,320 to Williams et al.
In a rising stem gate valve, the stem is attached to the gate in a manner which prevents axial movement of the stem relative to the gate. A mechanism is then provided for selectively driving the stem up and down in order to open and close the valve. On this type of gate valve, the backseat is set by moving the stem and the gate upwards until the stem backseats against the bonnet. Such valves may be automatically or remotely actuated, such as by a hydraulic cylinder. Alternatively, these valves may be manually actuated by providing a transmission means to convert the rotational motion of a handwheel into axial motion of the stem.
One such transmission means is a direct threaded connection between the handwheel and the stem. Unfortunately, for many large or high pressure valves which require large actuating forces, this method requires more torque to be applied to the handwheel than is practical to exert by hand. When the valve is closed, the entire upstream side of the gate is exposed to the full working pressure of the fluid while a portion of the downstream side of the gate is often at ambient pressure. This pressure differential results in very high forces which push the gate against the downstream seat. This engagement between the gate and the downstream seat in turn creates large gate-to-seat drag forces which must be overcome when gate is moved from the closed position to the open position. Another force which must be overcome is the drag which the stem packing exerts on the stem.
Rising stem gate valves can be further divided into two types: (a) balanced stem gate valves and (b) un-balanced stem gate valves. In a balanced stem gate valve, a second stem is attached to the gate at the end opposite the first stem. An example of this type of gate valve is shown in U.S. Pat. No. 4,230,299 to Pierce, Jr. It will be appreciated that when pressurized fluid is present in the gate cavity, a force is exerted on each stem which is equal to the product of the pressure and the cross-sectional area of the stem where it passes through the stem packing. In a balanced stem gate valve, the forces acting on the two stems will cancel each other out, resulting in substantially zero (or a balanced) net force to overcome when moving the gate. The disadvantages of balanced stem gate valves include increased cost and complexity and the creation of an additional potential leak path between the second stem and its corresponding stem packing.
An example of an unbalanced stem gate valve is disclosed in U.S. Pat. No. 4,569,503 to Karr, Jr. Although in this type of gate valve the unbalanced stem forces must be overcome when moving the gate, it will be appreciated that this design is simpler than the balanced stem gate valve. In the valve shown in Karr, Jr., the gate opening is disposed in the upper part of the gate, such that the valve is open when the gate is in its lowered position and closed when the gate is in its raised position. The disadvantage of this configuration is that when the valve is moved from the closed position to the open position, both the unbalanced stem force and the maximum gate-to-seat drag forces must be overcome simultaneously.
In order to overcome these combined forces and still maintain the required handwheel torque at an acceptable level, a transmission means which provides a substantial mechanical advantage must usually be utilized. Karr, Jr. provides a ball screw device for raising and lowering the stem. Other valves utilize bevel or worm gear reduction boxes. One disadvantage of these devices is that, in order to sufficiently reduce the required torque on the handwheel, the gear ratio must be very high. Consequently, a large number of turns is required to open or close the valve. Moreover, since the rate at which an operator can turn the handwheel is limited, the gate necessarily traverses very slowly from one position to the other.
This relatively slow traverse is especially troublesome when moving the gate from the closed position to the open position. As soon as the gate opening intersects the flow bore in the downstream seat, the gate-to-seat seal is broken and a high velocity jet of fluid is forced through the intersection area. In many cases, the fluid may contain abrasive particles which tend to erode the valve components during high velocity flow. The longer the intersection area remains small, the longer it takes for pressure to equalize on the opposites sides of the gate. Thus, the slower the gate moves to the open position, the greater the amount of erosion.
A further disadvantage of the gate valves shown in the Williams et al., Pierce, Jr. and Karr, Jr. patents is that these valves must be in the closed position in order to backseat the stem against the bonnet. Consequently, multiple actuations of the valve are required to ensure that both the gate cavity and the bonnet are at ambient pressure. Typically, the valve must first be actuated to the open position in order to bleed down the system pressure on both sides of the valve. Then the valve must be actuated to the closed position in order to backseat the stem against the bonnet.